This invention relates generally to digital waveforming and wave shaping techniques as applied to electronic musical instruments, and more specifically to a binary interpolator circuit for digitally reducing abrupt changes in a slow varying portion of a waveform.
While the present invention may find use in a variety of applications, the description is facilitated by addressing the specific problem of binary interpolation of the decay portion of an envelope waveform of a note produced by an electronic musical instrument, and more specifically to an envelope waveform of a percussion voice, such as piano, of such an instrument. The invention is equally applicable to any data processing system where it is desired to reduce data transitional step sizes.
Many digital electronic processing techniques are currently being used in electronic musical instruments. Such digital techniques have proven useful in performing numerous relatively complex functions in the generation of electronic musical effects approximating very closely the characteristic sounds of a number of acoustic instruments, as for example a piano. Moreover, as digital electronic components become increasingly available both in conveniently small package sizes and at relatively low cost, the utilization of such components has also led to increased savings in both cost and size of the finished products. In electronic musical instruments, such components perform relatively complex functions to more accurately simulate sounds of acoustic musical instruments.
One such electronic tone generating system is described, for example in U.S. Pat. No. 4,067,253. In this patent, a specific system is described for simulating the sound of a piano, in accordance with the relative force with which the keys of the instrument are struck. Broadly speaking, a digital or stepwise changing waveform is produced simulating the characteristic envelope curve of a percussive type note as is played by a conventional piano. When this stepwise changing digital waveform is fed to audio output circuits, it may be directly converted to analog form by an integrator or smoothing circuit which generally "smooths" the transitions between adjacent steps of the digital waveform. It wil be noted that in a stepwise changing digital waveform wherein the steps are of a relatively short time duration, ie. in the range of 30 to 40 milliseconds apart, that changes in amplitude between adjacent steps are generally not audibly noticeable. This is in part owing to the natural tendency of conventional amplifier circuits and speakers to respond somewhat more slowly than the abrupt changes of the digital waveform fed to their inputs. Another contributing "smoothing factor" is the tendency of the human ear to integrate or smooth rapid amplitude transitions somewhat. However, in a percussion voice envelope, such as a piano, there is a relatively long decay portion, wherein the digital steps or changes in amplitude may be spaced in time on the order of 300 to 400 milliseconds. During this portion of the waveform, primarily due to the longer spacing in time between amplitude changes, all but very slight amplitude changes are generally audibly noticeable.
Accordingly, the binary interpolator circuit according to this invention is adapted to modify these relatively long steps, or widely spaced amplitude changes, so as to produce a plurality of smaller steps, both in time and in amplitude. The end result thus is the same amplitude variation, covered over the same period of time but, comprising smaller amplitude, more closely spaced steps, so as not to be audibly noticeable.